By: Barbara Ziesman, AAg, Provincial Specialist, Plant Disease
Crop rotation has an important role in an integrated approach to disease management. Diverse crop rotations can be used to break disease cycles, and reduce pathogen levels and disease pressure between host crops. When the pathogen survives in the field (either in the soil or on infected residue), time away from a host crop will provide time for the infected residue to decompose and for the number of living pathogen spores or resting structures in the soil to decline. This means that there will be lower levels of the pathogen present to cause disease the next time a susceptible crop is grown.
Crop rotation as a disease management tool will have the most significant impact when:
- The disease has a narrow host range. This means there are more options to select non-host crops to add to the crop rotation to provide time for pathogen levels to reduce. For example, crop rotation is a very effective management strategy for diseases like blackleg, clubroot, ascochyta blight and aphanomyces root rot, as these diseases only infect a small number major crops in Saskatchewan. On the other hand, crop rotation will be less effective for sclerotinia diseases, as the pathogen has the ability to cause disease in more than 400 plant types, including most of the broad-leafed crops grown in Saskatchewan.
- The pathogen overwinters in the field (in the soil or on infected residue) and spore dispersal only occurs over short distances. When the pathogen overwinters in the field, strategies such as crop rotation can directly impact the amount of pathogen present to cause disease. However, when the pathogen is primarily introduced through other means, crop rotation may have less of an impact. An example of this is stripe rust of cereals. The stripe rust pathogen does not typically overwinter in Canada, and the pathogen is often introduced to a field via spores carried on wind currents. Since the spores are introduced through an external source, crop rotation within a specific field will not impact disease levels in a given year.
The length of crop rotation needed will depend on how the pathogen survives in the field and the half-life of the spores or overwintering structures of the pathogen. As a general rule of thumb, a four-year crop rotation is recommended. However, a three-year crop rotation can be very effective for some diseases (e.g.: Fusarium head blight or clubroot), and a longer crop rotation is needed for others (e.g.: aphanomyces root rot of pea and lentil, where an eight-year rotation is recommended).
The chart below* illustrates some of the major and economically important diseases of Saskatchewan crops and the major crop types that they can infect. This can be used to identify crop rotations to provide adequate disease breaks to reduce pathogen levels and disease pressure.